1. Field of the Invention
The present invention relates to a process for producing aluminum oxide fibers, and more particularly to a process for producing aluminum oxide fibers that have a high aspect ratio and can be accurately aligned.
2. Description of the Prior Art
Ceramic fiber composite materials have been extensively utilized because they have, regarding mechanical properties, strength comparable to high-strength metallic materials and, as members, are lightweight and inexpensive. The development of these ceramic fiber composite materials has been mainly directed to an increase in heat resistance/strength and an increase in elasticity, or a mass production technique that can bring physical properties, including these properties, close to the theoretical values and can control the physical properties as desired.
Among others, aluminum oxide whiskers, as compared with the other materials, have particularly higher strength and higher elasticity and better stability by virtue of higher melting point. In the production process thereof, the relationship between the crystal growth direction and the strength level has been established. On the other hand, for continuous fibers of .alpha.-type alumina, the strength level is lower than that of whiskers, but it is still considerably higher than that for other continuous long fibers.
Conventional aluminum oxide whiskers have been produced mainly by a gaseous phase process. A reaction involved in this process is such that Al.sub.2 O.sub.3 is reduced with H.sub.2 at a high temperature of 2000.degree. C. to evolve an Al.sub.2 O gas which is then grown into whiskers at 1600 to 1700.degree. C. Other reaction methods reported in the art include a reaction of molten aluminum with a hydrogen stream containing H.sub.2 O at 1300 to 1450.degree. C., a reaction of an AlCl.sub.3 gas with an H.sub.2 O gas at 1500.degree. C. or above, and a reaction of an AlF.sub.3 gas with an H.sub.2 O gas at 1400.degree. C.
On the other hand, no significant variation in strength and a high aspect ratio are required of fibers as a material for fiber composite materials (MMC (metal-base metal composite materials) and FRP (fiber-reinforced plastics). The conventional fibers, however, cannot satisfy these requirements. Accordingly, the development of the above-described aluminum oxide fibers improved in the above properties have been desired in the art.